Sulphur isotope abundances in Aphebian clastic rocks: implications for the coeval atmosphere

Abstract

The evolution of the atmosphere has been of interest to many scientists because it relates to the history of the ocean, the sedimentary cycle, the geochemical cycle of volatile elements, and the development of life. It also has a key role in the genesis of iron formations and gold and uranium deposits. Although it is generally accepted that the primitive atmosphere was reducing, a debatable aspect is when and how the oxygen content increased. A rise of atmospheric oxygen pressure about 2,000 Myr BP is suggested by the appearance of redbeds and the banded iron formations^1,2. On the other hand, high atmospheric oxygen in very early Precambrian time, as a result of photodissociation of water, has been proposed³. This is supported by occurrences in Precambrian rocks of sulphates^4,5, volatile elements⁶, detrital minerals^5,7 and palaeosols⁸, which are similar to those found in younger rocks. Because the change of sulphur valency is often accompanied by isotopic fractionation, sulphur isotope abundances in minerals have been used to infer the environment at the time of their formation. It occurred to us that additional sulphur isotope analyses of pyrite from continental sedimentary rocks might provide evidence relative to the question of oxygen development in the atmosphere. Accordingly, a major study of sulphur isotope abundances in pyrite from fluviatile sedimentary rocks in the Elliot Lake area- Canada, and for comparison, data from occurrences in Northwest Territories, Canada, Witwatersrand, South Africa, and Jacobina, Brazil, are now reported and implications respecting the coeval atmosphere discussed.